Male dimorphism in three sympatric species of Onthophagus (Coleoptera: Scarabaeidae).

Onthophagus trituber, O. taurinus, and O. proletarius, three sympatric dung beetle species native to southern Taiwan, exhibit normally distributed body size (pronotum width) but non-normally distributed horn length due to the presence of horned and hornless morphs. The scaling relationships between horn length and body size were established by using sigmoidal and segmented linear estimation, and the horned/hornless ratio in each of the species was estimated. The ratios estimated by sigmoidal curves showed that all three species were biased toward the hornless morph, whereas the ratios from segmented lines showed that only one species (O. taurinus) was biased toward the hornless morph. Nevertheless, the results of the two methods of estimation were concordant in ranking the horned morph proportions among the three species; O. proletarius had the largest proportion of horned morphs, whereas O. taurinus had the smallest, suggesting that the fewer horned morph in O. taurinus likely resulted from both intra- and inter-specific competition.